This invention relates to transmission systems, and more particularly to a continuously variable hydraulic transmission system.
The purpose of any transmission system, including a continuously variable transmission system, is to change the gear ratio of engine to wheels. For each set of engine parameters, such as engine RPM, vehicle speed, and accelerator and brake position, maximum engine efficiency is achieved at only one gear ratio. Variations in load and the finite number of gear steps in a conventional transmission result in less than optimum engine efficiency. Continuously variable transmissions, however, allow the engine to be operated at the most efficient speed.
Continuously variable transmission systems are known in the prior art. Prior art continuously variable transmission systems are generally of three types, each with advantages and limitations. The first type has a belt-driven configuration in which the transmission system includes metal-banded belts that transmit drive torque. This type of transmission is limited by the tensile strength of the steel bands and is used typically in low torque engines.
A second type of continuously variable transmission system has two sets of planetary steerable rollers housed between an inner and outer toroidal shaped disc, one driving and one driven. By tilting the steerable rollers, the relative diameters of engagement of the input and output toroidal discs can be varied to achieve a desired speed ratio. Because of very high contact pressures in such transmissions, ordinary transmision oil cannot be used and a high shear stiffness traction fluid is used to prevent metal to metal contact. The complexity and cost of the toroidal disc transmissions have so far precluded widespread use. The third type of continuously variable transmission system found in the prior art is typically limited to heavy duty applications because of cost and weight. This third type is basically a hybrid of a toroidal transmission and a hydrostatic transmission that can transfer power in any of three modesxe2x80x94purely hydraulic, combined hydraulic and mechanical, or purely mechanical.